Device for generating an ion beam

ABSTRACT

An ion beam generation device including an ion source, an extraction mechanism for ions emitted by the source, an accelerating mechanism of the ions thus extracted, a selector for the ions thus accelerated, and an electrostatic optical system for focusing the selected ions along a first axis. Further, a mechanism varies the distance between the ion source and the extraction means, this distance being counted along a second axis parallel to the first axis and constituting the axis of the ion beam emitted by the source. The device may be particularly applied to the manufacture of nanostructures.

TECHNICAL FIELD

The present invention relates to an ion beam generation device and alsoa method of control of this beam.

This invention is particularly applied to the manufacture of structureshaving very small sizes, less than 50 nm, and more particularly to themanufacture of nanostructures having sizes of the order of 10 nm orless.

The invention finds application in various fields such as electronics(particularly relating to solely electron devices, for exampletransistors), data storage at ultra-high density (using nanostructuresformed on magnetic materials), and ultra-high speed semiconductordevices (using nanostructures formed on semiconductor materials).

We indicate from now on that the present invention preferably uses apoint ion source, that is, an ion source with a very bright, pointemissive zone.

Moreover, this point ion source is preferably an LMIS, that is, a liquidmetal ion source.

PRIOR ART

A liquid metal ion source is known from the following document, which isincorporated herein by reference: [1] International application PCT/FR95/00903, international publication number WO 96/02065, invention ofJacques Gierak and Gérard Ben Assayag, corresponding to U.S. Pat. No.5,936,251.

The source described in this document [1] is an example of a sourcewhich can be used in the present invention.

An ion beam generation device, comprising a liquid metal ion source aswell as an asymmetric three-element electrostatic lens system, is knownfrom the following document: [2] U.S. Pat. No. 4,426,582, invention ofJ. H. Orloff and L. W. Swanson.

The device known from document [2] has a disadvantage: it does notpermit separation of the ion extraction function and the ionacceleration function.

Moreover ion beam generation devices are known in which are aligned anion source, a diaphragm, and electrostatic lenses for focusing by meansof appropriate displacements of the ion source and the diaphragm.

These known devices, termed FIB and producing focused ion beams, do notpermit the manufacture of nanostructures of good quality and of sizesless than 50 nm.

Moreover, a device for controlling the shape of a focused ion beam isknown from the following document: [3] U.S. Pat. No. 4,704,526,invention of H. Kyogoku and T. Kaito (Seiko Instruments and ElectronicsLtd.).

The control method known from this document [3] is only a transpositionof the control method conventionally used in scanning electronmicroscopy or in electron beam lithography.

Such a technique cannot be used in the nanometer field.

Moreover, this known method necessitates the prior formation ofexpensive and fragile calibration markers which cannot be reused.

SUMMARY OF THE INVENTION

The present invention has the object of remedying the previousdisadvantages.

In contrast to the device known from document [2], the device of theinvention permits separating the ion extraction function from the ionacceleration function.

Moreover, the invention uses a technique of aligning a diaphragm andelectrostatic lenses along a mechanically perfect axis, a techniquewhich leads to much better performance than that of the known FIBdevices mentioned above: the present invention permits the manufactureof good quality nanostructures of sizes less than 50 nm.

Furthermore, the ion beam control method which is the subject of theinvention is much more precise than the technique known from thedocument [3], and does not necessitate the prior production of expensiveand fragile calibration markers.

The method of the invention may advantageously be used in any system ofnano-manufacture by FIB and is in particular applied to the control ofion beam generated by the device of the invention.

More precisely, the present invention has as its object an ion beamgeneration device characterized in that it comprises an ion source, anextraction means for ions emitted by the source, an acceleration meansfor the thus extracted ions, a selection means for the ions thusaccelerated, and an electrostatic optical system intended to focus thethus selected ions along a first axis, and in that the devicefurthermore comprises a means for varying the distance between the ionsource and the ion extraction means, this distance being taken along asecond axis which is parallel to the first axis and constitutes the axisof the ion beam emitted by the source.

According to a preferred embodiment of the device of the invention, theion source is a point ion source.

Preferably, this point ion source is a liquid metal ion source.

According to a preferred embodiment of the invention, the ion extractionmeans and the ion acceleration means are mutually independent and areseparately controlled by the application of respective variablevoltages.

Preferably, the ion selection means comprises a means of selecting onediaphragm among a plurality of diaphragms and of placing the selecteddiaphragm on the first axis.

The device of the invention furthermore comprises, preferably, a meansfor displacement of the source parallel to the ion selection means, thisdisplacement means being provided for bringing the first and second axesinto coincidence.

The present invention likewise has as its object a control method for anion beam emitted by an ion beam generation device toward a target andcapable of eroding this target, this method being characterized in thatit comprises a step of etching a test pattern on the target according toa predetermined digitized reference pattern, a step of forming adigitized image of the etched test pattern, and a step ofdifferentiating between this digitized image of the etched test patternand the digitized image of the predetermined reference pattern, and inthat these steps are repeated after modifying at least one controlparameter of the device, until suitable control is obtained.

According to a first particular embodiment of the method of theinvention, the image of the test pattern is formed by means of thedevice, by then causing the latter to operate in a scanning ionmicroscope mode, and collecting the secondary electrons generated bysweeping the target with the ion beam, and the obtained image isdigitized, and the steps are repeated after modifying at least onecontrol parameter of the device, until the digitized image of the testpattern coincides with the digitized image of the reference pattern.

In this case, the control parameter(s) may be focusing parameters orastigmatism correction parameters.

According to a second particular embodiment of the method of theinvention, the test pattern is a set of lines of predetermined length,formed by keeping the ion beam in a fixed position and displacing thetarget, and the image of the test pattern is formed by scanning ionmicroscopy and then digitized, and the parameters are gain parameters,in order to calibrate the size of the writing field.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood on reading thedescription of embodiments given hereinafter, purely as illustrationsand in no way limitative, and with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic sectional view of a particular embodiment of thedevice of the invention, and

FIG. 2 is a partial schematic perspective view of the device of FIG. 1,showing the strip forming a diaphragm support used in the device of FIG.1.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

FIG. 1 is a schematic view of a particular embodiment of the ion beamgeneration device of the invention.

The device of FIG. 1 comprises an ion source 2 for producing the ionbeam, as well as an extraction and acceleration system 4 for the ionbeam produced, this system comprising an extraction electrode 6 for thebeam produced and an electrode 8 for the final acceleration of the beamextracted. This electrode 8 constitutes the input of the electrostaticoptical system 10 which the device of FIG. 1 likewise comprises.

The support 12 of the ion source 2 may likewise be seen in this figure.

This source 2 is a conventional liquid metal ion source, for example forforming a gallium ion beam 14.

However, in the present invention, another LMIS could be used, forexample of the kind described in document [1], to form an aluminum ionbeam, for example.

Returning to FIG. 1, the conventional ion source 2 comprises aconductive rod 16 ending at a point, and also a conductive filament 18having coils 20 through which the point passes. In a known manner, therod is held by a pair of jaws 22. Conductive elements 24 and 26 forholding the ends of the filament may also be seen. A base 28 on whichthe support 12 for the source 2 is mounted may also be seen.

The extraction electrode 6 is provided with an extraction diaphragm 30and with holes such as the hole 32, permitting residual gas to be pumpedout when the device is to operate in vacuum.

The extraction and acceleration system 4 used in the device of FIG. 1 isoriginal because of its geometry. The extraction electrode 6 for thebeam 14 does not intercept the beam 14 at any point. The characteristicextraction voltage V0 for ion emission is controlled in the exampleconsidered by mechanically modifying the distance D between the source 2and the extraction electrode 6, this distance being taken parallel tothe axis Z of the ion beam 14 emitted by the source 2.

To do this, means are used for displacement of the extraction electrode6 with respect to the source 2 (or of the source with respect to thiselectrode). This displacement means is symbolized by the arrow F in FIG.1. It may be produced by those skilled in the art (and consists, forexample, of a mechanical means based on a threaded rod).

Furthermore, the extraction electrode 6 and the final accelerationelectrode 8 are independent and may be separately controlled byapplication of variable voltages.

The extraction and acceleration system 4 is likewise original by virtueof its optical characteristics. In operation, a voltage V1 is applied tothe source 2 and a voltage V2 to the extraction electrode 6. When thedifference V1−V2, equal to V0, is of the order of 10 kV, a beam of Ga⁺ions is emitted having an angular density of the order of 20 μA per unitsolid angle (at the level of the axis Z of the beam, forming a centralaxis of revolution).

If V0 is increased to about 17 kV, the angular density measured underthe same conditions reaches about 80 μA/steradian.

The system thus permits considerably increasing the useful brightness ofan LMIS, and this constitutes one of the crucial points for theapplication of the FIB technique to nano-fabrication.

The extraction and acceleration system is likewise original because ofits operating characteristics which make possible improved use of theoperation of the source 2.

Each electrode 6 and 8 may be controlled in an independent and specificmanner by a suitable high voltage generator (for example of the order of20 kV to 40 kV) (not shown).

Ion emission is controlled by a first high voltage supply (not shown)which is specifically optimized for regulating the ion beam currentemitted by the source 2. The value of this current is servo-controlledby feedback by means of an analog device optimized to have a rapidresponse dynamic. This servo-control is obtained by modifying the valueof the voltage applied to the source 2 around a value permitting ionemission by field evaporation to be maintained (of the order of 10⁹V/m).

It may be added that the emitted current may be known by means of amicroammeter which is placed in the electrical supply permitting thepoint of the source to be polarized. As an alternative, to know thiscurrent, it is possible to measure the current drawn by the accelerationelectrode 8.

The final ion energy is obtained by bringing the final accelerationelectrode 8 to a voltage V2 positive with respect to ground. Thispotential V2 is controlled with very high stability. This stability isensured by a generator (not shown) whose stability level is of the orderof 10⁻⁶. For example, a chopping type supply is used, having such astability level.

The extraction and acceleration system 4 of the ion beam 14 operateswith a performance better than that of the conventional extraction andacceleration systems.

The device of FIG. 1 also comprises a system 34 (FIG. 2) for selectingthe optical aperture of the focused ion column of the device, thiscolumn comprising the source 2, the source support 12, and theelectrostatic optical system 10.

The optical aperture selection system 34 comprises an assembly includinga bushing 36 provided for receiving a strip 38, controlled mechanicallyor electromechanically as symbolized by the arrow F1, this strip 38carrying a plurality of calibrated diaphragms 40.

This strip 38 permits placing a calibrated diaphragm 40, chosen fromamong the calibrated diaphragms carried by the strip 38, on the centralaxis Z1 of the electrostatic optical system 10, by means of atranslation along an X-axis perpendicular to this axis Z1. Thesediaphragms define different aperture values for the electrostaticoptical system 10 of the focused ion column.

Likewise on FIGS. 1 and 2 there is defined a Y-axis perpendicular to theX-axis and also to the Z-axis, which is parallel to the Z1-axis.

The system for optical aperture selection 34 is original by reason ofits general purpose.

Differing from all the known systems for optical aperture selection,this system used in the device of FIG. 1 does not have the purpose ofpermitting an alignment between a source, a diaphragm, and electrostaticlenses where the movable means to be “centered” along the X-axis and theY-axis are the diaphragm as well as the ion source. Such a known systemin fact only permits disposing plural easily interchangeable diaphragmshaving different sizes or the same size.

In the case of the device of FIG. 1, the centering of the essentialelements of the electrostatic optical system 10, namely the assemblyformed by the diaphragm 40 and by the electrostatic lenses 42 and 44,which will be returned to hereinafter, is ensured by close machiningtolerances. Thus, the input diaphragm of the electrostatic and theassociated lenses 42 and 44 are precisely aligned along one and only oneoptical axis Z1.

The principal purpose of this is to limit the optical aberrationsinduced by de-centering defects. These defects constitute theexperimental limit of all known electrostatic optical systems.

Returning to the electrostatic lenses 42 and 44 used in the device ofFIG. 1, it is stated that they are two in number, the first 42 beingseen in very schematic section in the plane of FIG. 1, while the second44 is seen from outside. Various holes 46 are also seen with which thelenses are provided and which in particular permit evacuating theelectrostatic optical system 10.

So as to be able to replace the diaphragm 40 (FIG. 1) and to positionthe replacement diaphragm along the X-axis with an uncertainty less thana tenth of a micrometer, a prior calibration is necessary of theabsolute position, along X, of the center of each diaphragm. A benchcheck on an optical check bench permits knowing precisely the positionsof the different diaphragms.

The only really necessary adjustment in the case of the device of FIG. 1consists of a displacement of the support 12, bearing the source 2,parallel to the input plane of the electrostatic optical system 10 andthus parallel to the diaphragm 40 (FIG. 1), this plane being parallel tothe X and Y axes. This permits aligning the central axis Z of theemission core of the ion beam 14 with the optical axis Z1 of theelectrostatic optical system 10.

To do this the base 28, which carries the source support 12, is providedwith micrometer plates symbolized by dot-dash lines 48, permittingdisplacing this base and thus the support 12 along the X- and Y-axes.

With respect to the known systems, the optical aperture selection systemof the device of FIG. 1 fulfills the following functions:

-   -   defining with the best possible precision a one and only optical        axis passing through the center of the input diaphragm of the        electrostatic optical system and the center of the various        electrodes (not shown) defining the two electro-optical lenses,        and    -   permitting a very rapid change of the optical aperture of the        focused ion column.

It should be noted that disposing several diaphragms permits thepossibility of greater control of the electrostatic optical system andfurthermore to space apart the interventions for maintaining the device.

Methods conforming to the invention will now be considered, particularlymethods of focusing and adjustment of the size of a writing field onnano-manufacture focused by ion beam. These methods are important withregard to an effective automation of nano-manufacture by FIB.

The methods explained hereinafter aim at resolving the fundamentalproblem of controlling the parameters of an ion column providing an ionprobe (focused ion beam directed onto a target) on a tens of nanometersscale.

This ion probe is intended to form nanometer-size structures bycontrolled ion irradiation.

The geometric form (size), the aspect (more or less rapid decrease ofthe number of particles on departing from the central axis of the beam)as well as the spherical characteristic of the distribution in the ionprobe at the level of the target are preponderant. At a nanometer scale,the problems are all the more complicated.

The methods proposed here aim at permitting a rapid and very preciseadjustment of the profile of this ion probe in a manual mode (requiringintervention by users), automatic mode (not requiring any intervention)or semi-automatic mode.

The points concerned are:

-   -   focusing (concentration) of the ion beam by the action of        electrostatic lenses (which the beam generation device        comprises), at the level of a target or of a sample, in an        impact of nanometer dimensions,    -   the correction of defects of sphericity of the incident ion        probe, and    -   the calibration of the writing field (field addressable by the        beam under the action of electrostatic deflectors) at the        surface of the target so as to always know the relative position        thereof to about several nanometers.

It is added that the beam generation device comprises electrostaticdeflectors for deviating this beam, and that the particularelectrostatic optical system 14 of FIG. 1 contains scanning electrodes(not shown) which form the electrostatic deflectors.

Let us consider the problems to be solved.

The use of a focused ion beam in an impact of 10 nm for nano-manufactureapplications has specific constraints which are very different fromthose occurring, for example, in scanning electron microscopy:

-   -   The pulverizing effect due to the energetic ions bombarding the        target leads to a more or less long-term destruction of the        calibration structures (generally gold markers on silicon) which        are conventionally used, for example, in electron beam        lithography. Moreover, the calibrated structures of several tens        of nanometers are delicate to manufacture, very fragile, and        above all very expensive.    -   The period of use of the ion beam can reach several hours,        requiring periodic control of the characteristics of this ion        beam to limit the influence of drift and of transitory        instabilities.    -   The reduced working distances, which are necessary to obtain a        geometrical enlargement of the source impact less than unity,        further reduce the usable depth of field. Moreover, certain        samples compromise patterns of very different heights, so that        the patterns are not all situated at the ideal focal distance.    -   In this last case, an ion which falls on such a sample at the        level of the optical axis traverses a much shorter path than        another ion, deviated by several millimeters with respect to        this axis. This optical path difference causes the appearance of        defects or aberrations. To limit these aberrations to an        acceptable value, the size of writing is limited to a field of        the order of a hundred micrometers. Thus, without an attached        device, the technique of nano-manufacture by FIB can only form        small, elementary patterns.    -   When an ultra-precise displacement of the sample is used, the        possibility appears of connecting several sub-structures to        define a pattern of larger size. But this remains subordinated        to a rigorous calibration of the elementary writing field by        FIB. In fact, the most perfect correspondence possible between        the coordinates of points defined within a scan field and the        coordinates of displacement of the plate which carries the        sample is necessary.

All this is complex, because the scan of the ion probe is obtained bymeans of a CAO (computer assisted design) generator of thedigital/analog type, while the plate carrying the sample is piloted by aspecific and independent interface. It should also be noted that anyvariation of the distance between the sample and the ion column, of theenergy of the ions, or of the nature of the latter, modifies the valueof the amplitude of the scan field.

In the present invention, a method is proposed which is rapid, and iscapable of being automated to calibrate the optical system of an ioncolumn, by using the property of heavy ions such as ions of gallium orof other metals, for example aluminum, of locally etching the targetwhich they strike.

With this method, the same ion beam generation device is capable offorming its own calibration marks, then verifying them, in a completelyself-contained manner.

-   -   It is proposed first of all to use the erosive effect of the        incident ion beam produced by an ion beam generation device to        etch a simple structure according to a pattern predetermined by        CAO, for example of a square, simple hole (“spot”), or cross        type, in a “sacrificed” zone of the sample. After FIB etching,        this structure is then imaged by the same device, now operated        in MIB, or scanning ion microscopy, mode under the same        conditions without any modification, by simply collecting the        secondary electrons resulting from scanning the surface of the        sample with the ion beam. The MIB image obtained is digitized        and, on this digitized MIB image, corresponding to the        effectively etched structure, it is then possible to        informatically superpose the digitized initial predetermined        pattern (square, hole or cross, for example) and to        differentiate (digitally) the two images. In the case of a        square type pattern, for example, a defect due to poor focusing        can then be detected and then remedied by increasing step by        step the localizing effect of the lenses with which the beam        generation device is provided. The process may be automated for        different enlargements and may be repeated step by step, until        the digitized MIB image and the initial pattern coincide        perfectly.    -   The same erosive effect may also be made use of for correcting a        possible defect of sphericity, also termed an astigmatic defect,        at the level of the impact of incident ions. In this case,        piercing a single hole of the order of 10 to 20 nm permits        obtaining very rapidly, in several tenths of a second, a        faithful image of the imprint. If an elliptical aspect of the        impact is sound, always by comparison with an “ideal” reference        image (circular image), and according to the orientation of the        obtained ellipse, it is possible to begin a procedure of        correction and iterative tests until the satisfactory decision        criterion, established by the users, makes the informatic system        used leave this sequence (preferably automatically).    -   The calibration of the size of FIB writing field is the last        crucial point which it is possible to make use of, preferably in        an automated manner, with a method according to the invention.        This method consists firstly of etching lines (set of parallel        lines or set of crossed lines) having a length known with very        little uncertainty. To do this, the ion beam is kept in the        “spot” mode and does not scan the surface of the sample, while        the latter is displaced by means of the plate which supports the        sample, the measurement of the displacements of this plate being        performed by laser interferometry. In this case, the mechanical        precision may be as good as several nanometers (of the order of        10 nm to 5 nm).

In this case, the markers are not found by scanning the target surfacewith the ion probe, but solely by displacing this target, the centralaxis of the etching ion beam being kept fixed. An MIB image of thestructures thus manufactured then permits, after digitization of thisimage, adjusting the gain of the amplifier stage of the ion beamgeneration device so that a digital weight of one or more bitscorresponds to a known displacement (of a certain number of nanometers)at the level of the sample. The calibration of the scan field is theneffected with the technique of laser interferometry measurement, themost efficient technique at present known for measuring relativedisplacements, and furthermore, a technique used by the National Bureauof Measurements.

1. Ion beam generation device, comprising: an ion source configured toemit ions; ion extraction means for extracting ions emitted by the ionsource; ion acceleration means for accelerating ions thus extracted; ionselection means for selecting the accelerated ions thus accelerated; anelectrostatic optical system configured to focus the ions thus selectedalong a first axis; and distance varying means for varying a distancebetween the ion source and the ion extraction means to control anextraction voltage for ion emission of the device, the distance beingtaken along a second axis parallel to the first axis and thatconstitutes an axis of the ion beam emitted by the ion source.
 2. Deviceaccording to claim 1, wherein the ion source is a point ion source. 3.Device according to claim 2, wherein the point ion source is a liquidmetal ion source.
 4. Device according to claim 1, wherein the ionextraction means and the ion acceleration means are independent from oneanother and are further controlled separately of one another byapplication of respective variable voltages.
 5. Device according toclaim 1, wherein the ion selection means comprises means for selectingone diaphragm among a plurality of diaphragms and for placing theselected diaphragm on the first axis.
 6. Device according to claim 1,further comprising displacement means for displacing the ion source in adirection parallel to the ion selection means, and configured to bringthe first and second axes into coincidence.
 7. Method of calibrating anion beam generation device with a target configured to be eroded by thedevice, the method comprising: (a) etching a test pattern on the targetcorresponding to a digitized predetermined reference pattern; (b)forming a digitized image of the etched test pattern; (c) comparing thedigitized image of the etched test pattern and the digitized image ofthe predetermined reference pattern; (d) modifying of at least onecontrol parameter of the device, and (e) repeating steps (a) to (d), ifnecessary, until a suitable control of a geometry, size, or density ofthe ion beam is obtained.
 8. Method according to claim 7, wherein theetched test pattern is imaged by operating the ion beam generationdevice in a scanning ion microscope mode and collecting secondaryelectrons produced by scanning the target with the ion beam, and steps(a) through (d) are repeated, if necessary, until the digitized image ofthe etched test pattern coincides with the digitized image of thepredetermined reference pattern.
 9. Method according to claim 8, whereinthe at least one control parameter includes a focusing parameter. 10.Method according to claim 8, wherein the at least one control parameterincludes an astigmatism correction parameter.
 11. Method of calibratingan ion beam generation device with a target configured to be eroded bythe device, the method comprising: etching a test pattern on the targetby moving the target in a direction for a known displacement; forming adigitized image of the etched test pattern; and adjusting a gain of thedevice to correspond to a digital weight of one or more bits of thedigitized image of the etched test pattern such that the gaincorresponds to the known displacement or a fraction of the knowndisplacement.
 12. Ion beam generation device, comprising: an ion sourceconfigured to emit ions; an ion extraction unit configured to extractthe ions emitted by the ion source; an ion acceleration unit configuredto accelerate the ions thus extracted; an ion selection unit configuredto select the ions thus accelerated; an electrostatic optical systemconfigured to focus the ions thus selected along a first axis; and amechanism configured to mechanically modify a distance between the ionsource and the ion extraction unit to control an extraction voltage forion emission of the device, the distance being taken along a second axisparallel to the first axis and that constitutes an axis of the ion beamemitted by the ion source.
 13. Device according to claim 12, wherein theion source is a point ion source.
 14. Device according to claim 13,wherein the point ion source is a liquid metal ion source.
 15. Deviceaccording to claim 12, wherein the ion extraction unit and the ionacceleration unit are independent from one another and are furthercontrolled separately of one another by application of respectivevariable voltages.
 16. Device according to claim 12, wherein the ionselection unit comprises means for selecting one diaphragm among aplurality of diaphragms and for placing the selected diaphragm on thefirst axis.
 17. Device according to claim 12, further comprising adisplacement unit configured to displace the ion source in a directionparallel to a moving direction of the ion selection unit and configuredto bring the first and second axes into coincidence.
 18. Methodaccording to claim 7, wherein the device is the device of claim
 12. 19.Method according to claim 11, wherein the device is the device of claim12.